The stable products resulting from γ-irradiation of 2′-deoxycytidine in aerated aqueous solutions have been purified to homogeneity and characterized by MS and NMR analyses. The main group of products, which are similar in structure to those that arise from the • OH-mediated decomposition of thymidine, included four diastereomers of 5,6-dihydroxy-5,6-dihydro-2′-deoxyuridine (2-5), 5-hydroxy-2′-deoxycytidine (6), two diastereomers of N 1 -(2-deoxy-β-D-erythro-pentofuranosyl)-5-hydroxyhydantoin (7 and 8), and N 1 -(2-deoxyβ-D-erythro-pentofuranosyl)formamide (9). These products are likely formed by way of either intermediate 5( 6)-hydroxy-6(5)-peroxyl radicals or the corresponding hydroperoxides. In labeling experiments, 18 O 2 was incorporated into products 2-9 with about 30% label at C 5 and 60% at C 6 . Several other products were observed in the γ-irradiation of 2′-deoxycytidine in aerated aqueous solutions. They included four isomeric nucleosides of biuret (10-13), two diastereomers of N 1 -(2-deoxy-β-D-erythro-pentofuranosyl)-1-carbamoyl-2-oxo-4,5dihydroxyimidazolidine (14, 15), and aminocarbonyl[2-(2-deoxy-β-D-erythro-pentofuranosyl)amino]-2-oxoethylcarbamic acid ( 16). The first step in the formation of these products is probably intramolecular addition of the 5-hydroxy-6-hydroperoxide to C 4 of the pyrimidine ring, resulting in a 4,6-endoperoxide which decomposes into an open-chain R-hydroxyaldehyde by C 4 -C 5 cleavage. The formation of products 10-13 can be explained by subsequent fragmentation of the intermediate R-hydroxyaldehyde, whereas cyclization gives rise to products 14 and 15. In the case of product 16, a series of reactions have been proposed, starting with the rearrangement of the R-hydroxyaldehyde. The above pathways are supported by 18 O 2 -labeling experiments. Finally, we characterized two diastereomers of 5′,6-cyclo-5-hydroxy-5,6-dihydro-2′-deoxyuridine (17, 18) in the γ-irradiation of 2′-deoxycytidine in aerated aqueous solutions. These products likely arise from initial • OH abstraction of a proton from C 5′ of the sugar moiety, followed by intramolecular addition of the resulting radical to C 6 of the cytosine moiety. The • OH-mediated decomposition of 2′-deoxycytidine in DNA is discussed.

The mechanisms and products of photosensitized and singlet oxygen oxidation of nucleic acid model compounds have been investigated using kinetics analysis and product isolation and identification. A competitive kinetics analysis is presented as a simple yet quantitative method for the determination of the role of singlet oxygen in homogeneous chemical reactions. This method is compared to other techniques that have been used in the past including determination of the solvent isotope effect and the effect of singlet oxygen quencliers and variations in oxygen and substrate concentration. The results of competitive kinetics experiments indicate that the photooxidation of dGuo (2'-deoxyguanosine) at pH 7-10 in buffered aqueous solution sensitized by Rose Bengal, Methylene Blue, Thionine or several other dyes proceeds predominantly by a pathway requiring intermediate formation of singlet oxygen. In contrast, dGuo photooxidation under the same conditions sensitized by flavin or pterin derivatives proceeds predominantly by pathways that do not involve singlet oxygen. Most of the primary products of dGuo photooxidation are very unstable and consequently are difficult, if not impossible, to isolate and characterize. However, two compounds have now been isolated that correspond to the addition of a single molecule of oxygen to the purine ring of guanine with the purine ring system still intact. Some of the products formed in the singlet oxygen oxidation of 2'deoxyguanosine appear to be different than some of the products formed in flavin photosensitized reactions in agreement with kinetic results that indicate that the flavin sensitized reactions proceed by mechanisms that do not involve intermediacy of singlet oxygen. If the unique nature of the products of 2'-deoxyguanosine oxidation by singlet oxygen can be firmly established it may be possible to detect singlet oxygen reactions in v i m by product analysis.' Deceased.

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The near ultraviolet photolysis of an aerated aqueous solution of thymidine containing 2-methyl-1,4-naphthoquinone gives rise to two main classes of photoproducts as a result of the initial formation of a pyrimidine radical cation. These photo-oxidation products have been separated by high performance liquid chromatography and further characterized by various spectroscopic techniques including fast atom bombardment mass spectrometry and high field 1H and 13C nuclear magnetic resonance analysis. This photoreaction constitutes an excellent model to study the chemical properties of the thymidine radical cation which is expected to be one of the primary consequences of the direct effects of ionizing radiation.

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